The invention relates to a digital solid-state trip unit for an electrical circuit breaker with separable contacts.
Satisfactory operation of an electrical circuit breaker depends on the state of wear of the contacts, a poor contact causing overheating by Joule effect and destruction of the circuit breaker. Circuit breakers often comprise an insulated housing, notably a moulded case, which gives them great reliability, but this housing hampers users used to performing visual checks of the state of the circuit breaker contacts. Such checking is frequent in open type, low voltage circuit breakers with high ratings, which are arranged for disassembly and replacement of the worn contacts. It is important to detect contact wear in time to avoid the whole switchgear device being destroyed and this check must be easy and avoid, in particular, having to disassemble the parts.
Circuit breakers are often equipped with a counter indicating the number of operations and thereby the degree of mechanical wear of the device, but this indication is insufficient to know the wear of the contacts, an opening on a short-circuit gives rise to greater erosion of the contacts than that caused by a simple breaking of the rated current.
It has furthermore been proposed to check the state of a switchgear device by taking account of the current broken.
In a state-of-the-art device, a mechanical contact associated with the contacts of the switchgear device sends a read signal of a memory whose data input is connected to a current measuring device and whose output supplies a wear value associated with the current measured at the time of reading. The wear values read in the memory are added so as to supply a value representative of the degree of contact wear. If this type of device is used in conjunction with a circuit breaker, there can be a non-negligible time lag between the moment a tripping order is sent to the circuit breaker and the moment the contacts open, and it is obvious that the current value measured at the time of reading the memory does not correspond to the peak current value.
A device is moreover known wherein a microprocessor computes a value representative of the degree of contact wear from the current value i during breaking and from the number of breaks n forming the integral .intg.i.n.dt, and causing tripping of the circuit breaker when this value is greater than a preset threshold.